Electrostatic MEMS resonators have been a promising technological candidate to replace conventional quartz crystal resonators due to the potential for smaller size, lower power consumption and low-cost silicon manufacturing. Such devices typically suffer, however, from unacceptably large motional-impedance (Rx). MEMS devices operating in the out-of-plane direction, i.e., a direction perpendicular to the plane defined by the substrate on which the device is formed, have the advantage of large transduction areas on the top and bottom surfaces, resulting in a reduction in motional-impedances. Consequently, out-of plane devices have received an increasing amount of attention resulting in significant advances in areas such as digital micro-mirror devices and interference modulators.
The potential benefit of out-of-plane electrodes is apparent upon consideration of the factors which influence the Rx. The equation which describes Rx is as follows:
                    R        x            =                        c          r                          η          2                      ;    with      η    =                  V        ⁢                              ∂            C                                ∂            g                              =                                    ɛ            0                    ⁢          AV                          g          2                    wherein “cr” is the effective damping constant of the resonator,
“η” is the transduction efficiency,
“g” is the gap between electrodes,
“A” is the transduction area, and
“V” is the bias voltage.
For in-plane devices, “A” is defined as H×L, with “H” being the height of the in-plane component and “L” being the length of the in-plane component. Thus, η is a function of H/g and H/g is constrained by the etching aspect ratio which is typically limited to about 20:1. For out-of-plane devices, however, “A” is defined as L×W, with “W” being the width of the device. Accordingly, η is not a function of the height of the out-of-plane device. Rather, η is a function of (L×W)/g. Accordingly, the desired footprint of the device is the major factor in transduction efficiency. Out-of-plane devices thus have the capability of achieving significantly greater transduction efficiency compared to in-plane devices.
Traditionally, out-of-plane electrodes are not fully utilized because of the difficulty in reliably fabricating such devices. For example, packaging is difficult for out-of-plane devices because out-of-plane electrodes are easily damaged during packaging processes. MEMS resonators incorporating an out-of-plane electrode are particularly challenging because such devices require a vacuum encapsulation process.
What is needed therefore is a simple and reliable device with an out-of-plane electrode and method for producing the device. A device incorporating an out-of-plane electrode that is easily fabricated with an encapsulated vacuum would be further beneficial.